RF generator with commutation inductor

ABSTRACT

A radio frequency (RF) generator comprises a first half bridge including first and second power transistors; a second half bridge including first and second power transistors; an RF output node coupling output nodes of the first and second half bridges, the output node outputting RF signals to a load; positive and negative rails coupled to a power source; and a first commutation inductor provided to store energy to commutate at least one of the half bridges.

The present applications claims the benefit of U.S. Provisional PatentApplication No. 60/575,435, filed on May 28, 2004, which is incorporatedby reference.

BACKGROUND OF THE INVENTION

The present invention relates to an radio frequency (RF) generator witha full bridge configuration.

A power amplifier or generator is a circuit for converting DC-inputpower into a significant amount of RF/microwave output power. There is agreat variety of different power amplifiers (PAs). A transmittercontains one or more PAs, as well as ancillary circuits such as signalgenerators, frequency converters, modulators, signal processors,linearizers, and power supplies. As used herein, the terms “powergenerator,” “RF generator,” and “power amplifier” are usedinterchangeably.

Frequencies from very low frequency (VLF) through millimeter wave (MMW)are used for communication, navigation, and broadcasting. Output powersvary from 10 mW in short-range unlicensed wireless systems to 1 MW inlong-range broadcast transmitters. PAs and transmitters are also used insystems such as radar, RF heating, plasma generation, laser drivers,magnetic-resonance imaging, and miniature DC/DC converters.

RF power amplifiers are commonly designated into various differentclasses, i.e., classes A-F. Classes of operation differ in the method ofoperation, efficiency, and power-output capability. The power-outputcapability (or transistor utilization factor) is defined as output powerper transistor normalized for peak drain voltage and current of 1 V and1 A, respectively.

FIG. 1 illustrates a basic single-ended power amplifier 100. The poweramplifier includes an active device 102, DC feed 104, and outputfilter/matching network 106. FIGS. 2A-2F illustrate drain voltage andcurrent waveforms of selected ideal power amplifiers. FIG. 2Aillustrates a wave form for a class A device. FIG. 2B illustrates a waveform for a class B device, and so on.

Generally, RF power amplifiers utilize a wide variety of active devices,including bipolar-junction transistors (BJTs), MOSFETs, JFETs (SITs),GaAs MESFETs, HEMTs, pHEMTs, and vacuum tubes. The power-outputcapabilities range from tens of kilowatts for vacuum tubes to hundredsof watts for Si MOSFETs at HF and VHF to hundreds of milliwatts for InPHEMTs at MMW frequencies. Depending upon frequency and power, devicesare available in packaged, chip, and MMIC form. RF-power transistorsgenerally are n-p-n or n-channel types because the greater mobility ofelectrons (versus holes) results in better operation at higherfrequencies.

While the voltages and currents differ considerably, the basicprinciples for power amplification are common to all devices. In class-Aamplification, the transistor is in the active region at all times andacts as a current source controlled by the gate drive and bias. Thedrain-voltage and drain-current waveforms are sinusoids, as shown inFIG. 2A. This results in linear amplification. The DC-power input isconstant, and the instantaneous efficiency is proportional to the poweroutput and reaches 50% at PEP. For amplification of amplitude-modulatedsignals, the quiescent current can be varied in proportion to theinstantaneous signal envelope. The utilization factor is ⅛. Class Aoffers high linearity, high gain, and operation close to the maximumoperating frequency of the transistor.

FIG. 2B illustrates drain voltage and current waveforms of a class Bdevice. The gate bias in this device is set at the threshold ofconduction. The transistor is active half of the time, and the draincurrent is a half-sinusoid. Since the amplitude of the drain current isproportional to drive amplitude, class B provides linear amplification.For low-level signals, class B is significantly more efficient thanclass A, and its average efficiency can be several times that of class Aat high peak-to-average ratios (e.g., 28% versus 5% for ξ=10 dB). Theutilization factor is the same as in class A, i.e., ⅛. Class B is widelyused in broad-band transformer-coupled PAs operating at HF and VHF.

FIG. 2C illustrates drain voltage and current waveforms of a class Cdevice. The gate of a conventional class-C device is biased belowthreshold, so that the transistor is active for less than half of the RFcycle. Linearity is lost, but efficiency can be increased arbitrarilytoward 100% by decreasing the conduction angle toward zero. This causesthe output power (utilization factor) to decrease toward zero and thedrive power to increase toward infinity. A typical compromise is aconduction angle of 150° and an ideal efficiency of 85%. When it isdriven into saturation, efficiency is stabilized, and the output voltageis locked to supply voltage.

FIG. 2D illustrates drain voltage and current waveforms of a class Ddevice. Class-D devices use two or more transistors as switches togenerate square drain-voltage (or current) waveforms. A series-tunedoutput filter passes only the fundamental-frequency component to theload, resulting in a power outputs of (8/π²)V² _(DD)/R for thetransformer-coupled configuration. Current is drawn generally onlythrough the transistor that is on, resulting in a 100% efficiency for anideal power amplifier. The utilization factor (½π=0.159) is the highestof the different classes of power amplifiers. If the switching issufficiently fast, efficiency is not degraded by reactance in the load.

Generally, class-D devices suffer from losses due to saturation,switching speed, and drain capacitance. Finite switching speed causesthe transistors to be in their active regions while conducting current.Drain capacitances are charged and discharged generally once per RFcycle, resulting in power loss that is proportional and increasesdirectly with frequency. Class-D devices with power outputs of 100 W to1 kW are readily implemented at HF.

FIG. 2E illustrates drain voltage and current waveforms of a class Edevice. Class E employs a single transistor operated as a switch. Thedrain-voltage waveform is the result of the sum of the DC and RFcurrents charging the drain-shunt capacitance. In optimum class E, thedrain voltage drops to zero and has zero slope just as the transistorturns on. The result is an ideal efficiency of 100%, elimination of thelosses associated with charging the drain capacitance in class D,reduction of switching losses, and good tolerance of componentvariation. Optimum class-E operation requires a drain shunt susceptanceof 0.1 836/R and a drain series reactance 1.1 5 R. It delivers a poweroutput of 0.577V² _(DD)/R for an ideal power amplifier with autilization factor of 0.098. Variations in load impedance and shuntsusceptance cause the power amplifier to deviate from optimum operation,but the degradations in performance are generally no worse than thosefor classes A and B.

FIG. 2F illustrates drain voltage and current waveforms of a class Fdevice. Class F boosts both efficiency and output by using harmonicresonators in the output network to shape the drain waveforms. Thevoltage waveform includes one or more odd harmonics and approximates asquare wave, while the current includes even harmonics and approximatesa half sine wave. Alternately (“inverse class F”), the voltage canapproximate a half sine wave and the current a square wave. As thenumber of harmonics increases, the efficiency of an ideal poweramplifier increases from the 50% (class A) toward unity (e.g., 0.707,0.8165, 0.8656, 0.9045 for two, three, four, and five harmonics,respectively) and the utilization factor increases from ⅛ toward ½π. Therequired harmonics arise naturally from nonlinearities and saturation inthe transistor. While class F requires a more complex output filter thanother power amplifiers, the impedances at the “virtual drain” generallyneed to be correct at only a few specific frequencies.

Recently, high voltage MOSFETs, e.g., with 500V or 1000V MOSFETs, havebeen used in class “C” or “E” operation. However, the class C and Edevices are narrow band approaches because the square wave drive pulsesrequire a filter to remove unwanted spectral content. Efficiency is highbut power control is difficult. Power control is usually a variable DCpower supply which results in slow control of the output power anddifficulty in controlling power at low levels. It is possible to drivethese classes with a sine wave; however, the turn-on threshold varieswith the MOSFET die temperature which will change the conduction angle(pulse width) of the MOSFET, which can be problematic.

SUMMARY OF THE INVENTION

The present invention relates to an RF generator that has a full bridgeconfiguration. The full bridge configuration comprises high voltageMOSFETs that are operated using phase shift techniques. The MOSFETs areconfigured to handle 300 volts or more, or 500 volts or more, or 600volts or more, or 1000 volts or more according to applications. The RFgenerator is configured to operate in a range of 5 MHz to 50 MHz. In thepresent embodiment, the RF generator is configured to operate at anIndustrial Scientific and Medical (ISM) frequency, e.g., 13.56 MHz or27.12 MHz. In one implementation, the RF generator is a class D deviceand is configured to operate directly off line.

Generators with phase shift control have been used in recent years atfrequencies below 500 KHz. These power supply designs are at much lowerfrequencies than the present invention, which is in radio frequencies.Generally, it is difficult to increase the operating frequency ofbridges. Gate drive becomes very difficult for larger MOSFETs as theoperating frequency is raised. The drive current-can exceed 10 Amperepeaks for turn-on and turn-off. Unwanted resonances can occur due to thelarge gate capacitance and very small stray inductances. These unwantedresonances can result in uncontrolled turn-on or off of the MOSFETs andresult in failure of devices. At lower frequencies, hard switching athigh voltages is not a limiting factor. At high frequency, e.g., 13.56MHz, hard switching at high voltage is a serious concern as powerdissipation becomes excessive. Zero voltage switching becomes apreferred design. This operation is difficult to maintain over the fulloperating range of a power stage when phase is shifted.

The present embodiments use one or more commutation inductors to storesufficient energy to provide the energy needed to charge the outputcapacitance of the MOSFETs and provide RF generators with phase shiftcontrol. There are various advantages associated such RF generators: (1)may be operated in a broader frequency range, (2) may operate with fixedDC voltages that are not highly filtered, (3) eliminates the need for avariable DC power supply, and (4) may be operated at very low power tofull power.

In one embodiment, a radio frequency (RF) generator comprises a firsthalf bridge including first and second power transistors; a second halfbridge including first and second power transistors; an output nodecoupling the first and second half bridges and RF signals to a load;positive and negative rails coupled to an AC power source via arectifier; a first blocking capacitor provided between the positive railand the load; and a second blocking capacitor provided between thenegative rail and the load. The first and second blocking capacitors areconfigured to isolate the load from the AC power source.

In another embodiment, a radio frequency (RF) generator comprises afirst half bridge including first and second power transistors; a secondhalf bridge including first and second power transistors; an RF outputnode coupling output nodes of the first and second half bridges, theoutput node outputting RF signals to a load; positive and negative railscoupled to an AC power source via a rectifier; and a first commutationinductor provided to store energy and facilitate commutation of at leastone of the half bridges.

The first commutation inductor is provided between the first and secondhalf bridges, so that there is no potential difference across the firstcommutation inductor when the first and second half bridges are inphase.

The RF generator further comprises a capacitor provided between thefirst commutation inductor and the output node of the second halfbridge. The RF generator further comprises a second commutation inductorprovided proximate the first half bridge to commutate the first halfbridge.

The RF generator further comprises a first commutation circuit includinga second commutation inductor, the first commutation circuit beingprovided proximate the first half bridge to commutate the first halfbridge. The RF generator further comprises a second commutation circuitprovide proximate the second half bridge to commutate the second halfbridge.

In another embodiment, a radio frequency (RF) generator includes a firsthalf bridge including first and second power transistors; a second halfbridge including first and second power transistors; an RF output nodecoupling output nodes of the first and second half bridges, the outputnode outputting RF signals to a load; positive and negative railscoupled to an AC power source via a rectifier; and a first commutationinductor configured to store energy and facilitate commutation of atleast one of the half bridges, wherein the RF signals are output to theload using phase displacements of the transistors.

In yet another embodiment, a method for operating an RF generator havingfirst and second half bridges comprising a full bridge configuration,includes causing the first half bridge to output a first signal with afirst phase; causing the second half bridge to output a second signalwith a second phase; combining the first and second signals to providean RF signal to a load coupled to the RF generator; and storing energyin a commutation inductor to store energy to facilitate commutation ofat least one of the first and second half bridges.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a basic single-ended power amplifier 100.

FIG. 2A illustrates drain voltage and current waveforms of class Adevice.

FIG. 2B illustrates drain voltage and current waveforms of a class Bdevice.

FIG. 2C illustrates drain voltage and current waveforms of a class Cdevice.

FIG. 2D illustrates drain voltage and current waveforms of a class Ddevice.

FIG. 2E illustrates drain voltage and current waveforms of a class Edevice.

FIG. 2F illustrates drain voltage and current waveforms of a class Fdevice.

FIG. 3 illustrates an RF generator being coupled to an AC power sourceand a load.

FIG. 4 illustrates a more detailed view of the RF generator according toone embodiment of the present invention.

FIG. 5 illustrates an RF generator having a full bridge configurationaccording to one embodiment of the present invention.

FIG. 6 illustrates a schematic circuit showing locations of one or moreblocking capacitors.

FIG. 7A illustrates the waveforms when the half bridges are controlledto output full power.

FIG. 7B illustrates the waveforms when the MOSFETs are operated at about90 degrees out of phase.

FIG. 7C illustrates the waveforms when the MOSFETs are operated at 180degrees out of phase.

FIG. 8A illustrates an RF generator with a commutation inductoraccording to one embodiment of the present invention.

FIG. 8B illustrates a commutation inductor that is provided between thetwo half bridges and directly connected to their outputs.

FIG. 8C illustrates an RF generator with a commutation inductoraccording to one embodiment of the present invention.

FIG. 9 illustrates currents associated with the MOSFETs and according toone embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an RF generator that has a full bridgeconfiguration. The full bridge configuration comprises high voltageMOSFETs that are operated using phase shift techniques.

The present embodiment relates to an RF generator operating at an ISMfrequency, e.g., 13.56 MHz, as disclosed in U.S. patent application Ser.No. ______, entitled, “RF Generator With Reduced Size and Weight,” filedon May 27, 2005, assigned to the assignee of the present application,which is incorporated by reference. The RF generator uses a high voltagephase shift controlled full bridge. A full bridge design offers severaladvantages which are helpful in RF operation. These include a higherutilization of the MOSFET ratings. In a bridge design, the voltage islimited to the supply rail (e.g., positive rail), whereas it is not thecase in a class C or E, and thus must be designed with very largemargins in case of reflected power. Another advantage is the drive pulsewidth is fixed. Phase shift control allows the output power to becontrolled by the phase displacement between two half bridges. The twooutputs are summed to produce a single output which can be varied fromzero to full output power by controlling the phase difference betweenthe two half bridges. This allows power control with fixed supplyvoltage rails which because of the high operation frequency can beoperated directly off line by using blocking capacitors.

One advantage of using a phase shift design is the ability to varyfrequency. The high Q circuits used with classes C and E precludevarying frequency by any significant amount. The bridge circuit has asymmetry which results in an ability to adjust and thus reduce secondharmonic distortion. This allows for a different output network designwhich can provide for a wider frequency of operation because it does notrequire as much attenuation of the second harmonic.

In class C and E devices, the output power is typically control by usinga variable DC power supply. This limits the speed at which the outputpower can be varied to that of the DC power supply. The phase shiftcontrol limits the speed at which power can be varied only to the speedat which phase can be varied and the Q of the output network. Phase canbe varied at rates of 10 degrees per cycle or more and thus can resultin very high speed power control or pulsing.

Another characteristic of the phase shift is improved performance at lowpower. Conventional designs using class C or E have great difficultywhen the power supply voltage is reduced to low levels. This is due tothe large capacitances, at low drain voltages, in the MOSFET, allowinggate drive signals to be fed to the output through the Crss capacitance(drain to gate capacitance) and detuning of the output network with thevery large increase in the average output capacitance Coss. There areother advantages associated with the present RF generator, as will beappreciated by those skilled in the art.

FIG. 3 illustrates an RF generator 302 being coupled to an AC powersource 304 and a load 306. The power source is a typical AC source witha relatively low frequency, e.g., 60 Hz. The load is a device orequipment, e.g., a plasma chamber, that is run using the outputgenerated by the RF generator.

FIG. 4 illustrates a more detailed view of the RF generator 302according to one embodiment of the present invention. The RF generatorincludes a rectifier 402 that receives the AC current and converts itinto a DC current. The RF generator uses fixed DC voltages rather thanvariable DC power supply since phase shift technique is used. Generally,the rectifier includes a bridge configuration to convert the 60 Hz to aDC current. A phase shift RF power section 404 receives the DC currentand sends out an RF output according to the controls of a phase control406. The phase control comprises four gate drivers, each driving aMOSFET (see FIG. 5) that is arranged in a full-bridge configuration.

FIG. 5 illustrates an RF generator 502 having a full bridgeconfiguration according to one embodiment of the present invention. TheRF generator 502 includes first, second, third and fourth MOSFETs 504,506, 508, and 510. In the present implementation, the MOSFETs are“IXYS-RF MOSFET IXZ211N50,” but other types of power MOSFETs may be usedin other implementations. The first and third MOSFETs 504 and 508 definea first half bridge, and the second and fourth MOSFETs 506 and 510define a second half bridge.

First, second, third, and fourth gate drivers 512, 514, 516, and 518 arecoupled to the control terminals of the first, second, third, and fourthMOSFETs, respectively. The MOSFETs are configured to handle at least 500volts and at least 11 amperes in the present implementation. An AC powersource 520 is coupled to a positive rail 522 and a negative rail 524 viaa rectifier 521, defining a given potential difference V. The rectifieris provided between the AC power source and nodes 526 and 528 to provideDC currents to the node 526. The DC currents are supplied to the firstand second MOSFETs via the positive rail 522. A first capacitor C1 isprovided between the positive and negative rails. IN the presentembodiment, a fixed DC voltage is provided to the first and secondMOSFETs.

A resonant circuit 530 is provided between the output nodes of the firstand second MOSFETs, so that the RF generator can operate at resonatefrequency and avoid hard switching. The circuit 530 includes second andthird capacitors C2 and C3, and first, second, and third inductors L1,L2, and L3.

In the present implementation, the second and third capacitors havecapacitance of 5.1 nf each. The first and second inductors L1 and L2have inductance of 400 nH each. The third inductor L3 has inductance of40 nH. In other implementations, these components may have differentvalues.

The values of the inductors L1 and L2 have been selected to facilitatethe commutation of the MOSFETs, such that hard switching is avoided formuch of the phase shift range. Hard switching is not completely avoidedin the present embodiment because the currents in the inductors are notidentical as phase shift is varied. One of the half bridges would have areduced current as the phase is changed from zero of 180 degrees. Thereduction in current results in only a partial resonant commutation withthe remainder being hard switching.

An impedance matching circuit 532 is provided between the resonatecircuit 530 and a load 534 that is represented as a resistor R5. Thematching circuit includes a fourth inductor L4 and fifth and sixthcapacitors C5 and C6.

In the present implementation, the fourth inductor has inductance of 270nH. The fifth and sixth capacitors C5 and C6 have capacitance of 180 pfand 1.1 nf, respectively. These components may have different values indifferent implementations.

The RF generator 502 also includes a plurality of blocking capacitorsC2, C3, and C4 to isolate the load 534 from the power section andoperate the RF generator directly off line. The blocking capacitor orfourth capacitor C4 has capacitance of 5.1 nf in the presentimplementation but may have other values in other implementations.

To operate directly offline, at least two blocking capacitors are used.That is, at least one blocking capacitor 542 is provided between thepositive rail 522 and the load 534, as shown in FIG. 6. The capacitor542 corresponds to the blocking capacitor C2 or C3. At least anotherblocking capacitor 544 is provided between the negative rail 544 and theload 534. The capacitor 544 corresponds to the blocking capacitor C4.The great difference in frequency between the very high output frequency(e.g., 13.56 MHz) and the very low input frequency (e.g., 60 Hz) of theAC power source 520 enables the use of low frequency blocking capacitorsC2, C3, and C4 to isolate the load from the power section. This allowsthe output to be grounded without excessive current flow from the 60 Hzpower

In operation, the phase of the two half bridges of the RF generator 502is varied to control the power output. The output of the two halfbridges are combined using a network to sum the outputs into a singlenode 537. The single node is then impedance matched to the output usingthe matching circuit 532.

FIGS. 7A-7C illustrate the waveforms generated by the RF generator 502according to the present embodiment. These waveforms are illustrated asquasi-square waves for illustrative convenience. However, they are inreality closer to sine waves due to the filtering of the total network.

FIG. 7A illustrates the waveforms when the half bridges are controlledto output full power. A zero degree phase relationship is maintained forthis operation. A first waveform 702 illustrates the output of theMOSFET 504, and a second waveform 704 illustrates the output of theMOSFET 508. Similarly, a third waveform 706 illustrates the output ofthe MOSFET 506, and a fourth waveform 708 illustrates the output of theMOSFET 510. An output waveform 710 illustrates the power output of theRF generator that results from combining the outputs of the aboveMOSFETs. Since the MOSFETs are operated in phase, full power is output.The node 537 switches at full pulse widths similar to the drivewaveforms.

FIG. 7B illustrates the waveforms when the MOSFETs are operated at about90 degrees out of phase. A first waveform 712 illustrates the output ofthe MOSFET 504, and a second waveform 714 illustrates the output of theMOSFET 508. Similarly, a third waveform 716 illustrates the output ofthe MOSFET 506, and a fourth waveform 718 illustrates the output of theMOSFET 510. An output waveform 720 illustrates the output of the RFgenerator that results from combining the outputs of the above MOSFETs.The power output is lower since the MOSFETs are not being operated inphase, as shown by the smaller pulses.

FIG. 7C illustrates the waveforms when the MOSFETs are operated at 180degrees out of phase. A first waveform 722 illustrates the output of theMOSFET 504, and a second waveform 724 illustrates the output of theMOSFET 508. Similarly, a third waveform 726 illustrates the output ofthe MOSFET 506, and a fourth waveform 728 illustrates the output of theMOSFET 510. An output waveform 730 illustrates the output of the RFgenerator that results from combining the outputs of the above MOSFETs.Since the MOSFETs operated 180 degrees out of phase, no power is output.

Although there is no power output when the MOSFETs are operated in 180degrees out of phase, currents continue to flow through the inductors L1and L2. These inductors are being charged and discharged. The potentialof the node 537, however, does not change and remains at the same level.This is so since the inductors L1 and L2 are a voltage divider, eachwith the same inductance. The node 537 remains at V/2 (i.e., a half ofthe potential difference between the positive and negative rails 522 an524) as long as the drive is symmetrical.

FIG. 8A illustrates an RF generator 802 with a commutation inductor 804(or inductor L5) according to one embodiment of the present invention.The RF generator 802 has a full bridge configuration as with the RFgenerator 502, discussed above. Nodes 526 and 528 couple a DC source, asbefore, so the same numerals are used for illustrative convenience.

The communication inductor 804 is provided between the two half bridges806 and 808. The half bridge 806 includes an upper MOSFET 810 and alower MOSFET 812. The half bridge 808 includes an upper MOSFET 814 and alower MOSFET 816.

The commutation inductor 804 has one end connected to a node 805 that isprovided between a capacitor C2 and an inductor L2. Another end of thecommutation inductor is connected to a node 807 that is provided betweena capacitor C3 and an inductor L1. The inductor 804 has inductance of600 nH in the present embodiment. The communication inductor may haveother values in other implementations.

In phase controlled RF bridges, the current drops in one of the twobridges as phase is changed. At RF frequencies (2 MHz and up), MOSFETcommutation in a bridge circuit is of great concern. Hard switchingresults in very high power dissipation and low efficiency. Onecommonly-used method to prevent this is to resonant the output in a highQ network. This method, however, restricts the operation to a narrowfrequency and load range.

To avoid hard switching, a typical RF approach is to add a conjugate tocancel the output capacitance. This approach, however, may not besuitable as the average capacitance varies greatly as the power supplyrails are changed.

In the present embodiment, the commutation inductor/circuit 804 is addedto limit hard switching. The commutation inductor 804 stores energy forcommutating the half bridge. This inductor is provided across the phasecontrolled bridges and has the benefit of only being present, as far asthe circuit is concerned, when less than full output is selected (i.e.,the two half bridges are not in phase). At full output, there is nopotential difference across the inductor so no current flows in theinductor. When the two half bridges are not in phase, current will flowthrough the inductor and help maintain commutation current with theenergy stored in the inductor.

The commutation inductor may be arranged differently according toimplementations. For example, FIG. 8B illustrates a commutation inductor818 that is provided between the two half bridges and directly connectedto their outputs. A capacitor (not shown) may be provided between thecommunication inductor 818 and the output of one of the half bridges,e.g., the half bridge 808.

FIG. 8C illustrates an RF generator 822 with a commutation inductor 824according to one embodiment of the present invention. The RF generator802 has a full bridge configuration as with the RF generator 502,discussed above. Nodes 526 and 528 couple a DC source, as before, so thesame numerals are used for illustrative convenience.

The communication inductor 824 is provided between the two half bridges806 and 808. The commutation inductor 824 has one end coupled to a node823 and another end coupled to a node 825. The node 823 is connected toan output of the half bridge 806, and the node 825 is connected to anoutput of the half bridge 808. A capacitor 826 is provided between theinductor 824 and the node 825 as a DC blocking capacitor.

The RF generator 822 includes a communication circuit 828 that isconnected to a node 830 and a node 832. The commutation circuit includesa capacitor C7 and an inductor L6 in series. The capacitor C7 isproximate the node 830, and the inductor L6 is proximate the node 832.Alternatively, the capacitor C7 is proximate the node 832, and theinductor L6 is proximate the node 830. The node 830 is connected to theoutput of the half bridge 806. The node 832 is connected to the outputof the half bridge 832.

The commutation circuit 828 is used to store energy to facilitate thecommutation of the half bridge 806. Unlike the commutation inductor 824,the commutation circuit 828 is always present and is an additional loadeven at full power (zero phase condition). The series capacitor-inductor(or circuit 828) can be used on bridges which are not phase controlledto improve commutation.

FIG. 9 illustrates currents associated with the MOSFETs 810 and 814according to one embodiment of the present invention. The x-axisindicates the delay time in nanoseconds. The y-axis indicates the peakcurrents in amperes. A graph 902 shows the peak current of the MOSFET810 of the half bridge 806. A graph 904 illustrates the peak current ofthe MOSFET 814 of the half bridge 808. A graph 906 illustrates thecurrent output to the load by the RF generator. A graph 908 illustratesthe peak current flowing in the inductor L2. A graph 910 illustrates thepeak current flowing in the inductor L1. Adding one or more commutationinductors or circuits raises the current levels of the MOSFETs. That is,the graphs 902 and 904 are shifted upward.

The present invention has been illustrated in terms of specificembodiments to fully disclose and enable the invention. The embodimentsdisclosed above may be modified or varied without departing from thescope of the present invention. For example, a commutation circuit maybe provided proximate the half bridge 808 to assist in commutating thishalf bridge. The description and drawings provided herein, therefore,should not be used to limit the scope of the present invention.

1. A radio frequency (RF) generator, comprising: a first half bridgeincluding first and second power transistors; a second half bridgeincluding first and second power transistors; an RF output node couplingoutput nodes of the first and second half bridges, the output nodeoutputting RF signals to a load; positive and negative rails coupled toa power source; and a first commutation inductor configured to storeenergy and facilitate commutation of at least one of the half bridges,wherein the RF generator is operated using a phase shift control.
 2. TheRF generator of claim 1, wherein the first commutation inductor isprovided between the first and second half bridges, so that there is nopotential difference across the first commutation inductor when thefirst and second half bridges are in phase, wherein the power source isan AC power source, the positive and negative rails coupling the ACpower source via a rectifier.
 3. The RF generator of claim 1, furthercomprising: a capacitor provided between the first commutation inductorand the output node of the second half bridge, wherein the power sourceis an AC power source, the positive and negative rails coupling the ACpower source via a rectifier.
 4. The RF generator of claim 1, furthercomprising: a second commutation inductor provided proximate the firsthalf bridge to commutate the first half bridge.
 5. The RF generator ofclaim 1, further comprising: a first commutation circuit including asecond commutation inductor, the first commutation circuit beingprovided proximate the first half bridge to facilitate commutation ofthe first half bridge.
 6. The RF generator of claim 5, wherein the firstcommutation circuit further includes a capacitor in series with thesecond commutation inductor.
 7. The RF generator of claim 6, wherein thecapacitor of the first commutation circuit is provided between theoutput node of the first half bridge and the second commutationinductor.
 8. The RF generator of claim 5, further comprising: a secondcommutation circuit provide proximate the second half bridge tofacilitate commutation of the second half bridge.
 9. The RF generator ofclaim 1, wherein the first commutation inductor is provided proximatethe first half bridge to facilitate the commutation of the first halfbridge.
 10. The RF generator of claim 1, wherein the first and secondpower transistors are MOSFETs, and the third and fourth powertransistors are MOSFETs.
 11. The RF generator of claim 1, wherein the RFgenerator is configured to operate at an Industrial Scientific andMedical (ISM) frequency.
 12. The RF generator of claim 11, wherein theRF generator is configured to operate at 13.56 MHz.
 13. The RF generatorof claim 1, further comprising: a resonant circuit provided between thefirst and second half bridges.
 14. The RF generator of claim 13, whereinresonant circuit includes first, second, and third inductors, whereinthe first, second, and third inductors define a node where outputs ofthe first and second half bridges converge.
 15. The RF generator ofclaim 13, further comprising: a matching network provided between theresonant circuit and the load.
 16. The RF generator of claim 6, furthercomprising: a blocking capacitor provided between the positive rail andthe load.
 17. A radio frequency (RF) generator, comprising: a first halfbridge including first and second power transistors; a second halfbridge including first and second power transistors; an RF output nodecoupling output nodes of the first and second half bridges, the outputnode outputting RF signals to a load; positive and negative railscoupled to an AC power source via a rectifier; and a first commutationinductor configured to store energy and facilitate commutation of atleast one of the half bridges, wherein the RF signals are output to theload using phase displacements of the transistors.
 18. The RF generatorof claim 17 wherein the RF signals output to the load is at full powerwhen the first and second half bridges are in phase.
 19. The RFgenerator of claim 17, wherein the first commutation inductor isprovided between the first and second half bridges, so that there is nopotential difference across the first commutation inductor when thefirst and second half bridges are in phase.
 20. A method for operatingan RF generator having first and second half bridges comprising a fullbridge configuration, the method comprising: causing the first halfbridge to output a first signal with a first phase; causing the secondhalf bridge to output a second signal with a second phase; combining thefirst and second signals to provide an RF signal to a load coupled tothe RF generator; and storing energy in a commutation inductor to storeenergy to facilitate commutation of at least one of the first and secondhalf bridges.